This invention relates to microwave propagating devices capable of efficient energy transfer from a microwave generator to an ionizable gas to form a plasma. More particularly, this invention, which can used with both low power and high power microwave energy, relates to a cavity which includes a strip-line resonator for providing energy coupling between an electromagnetic field and a gas adapted to form a plasma.
In the prior art, a variety of devices have been developed to generate plasmas for various applications. In high power applications, typically, only three types of such devices are employed. The first is generally known as a one-electrode plasma torch, the second is a plasmatron which uses a rectangular wave guide in combination with a quartz discharge tube, and the third employs a circular cavity having a discharge tube located along the axis of symmetry of the cavity.
In low-power applications, the following cavity configurations have been generally utilized:
(1) a three-quarter wavelength Fehsenfield cavity; PA0 (2) one electrode coaxial plasmatron; PA0 (3) Benakker cavity of a radial-type TEM mode, and PA0 (4) surfatron type surface wave launcher.
These devices have been variously described in the following patent and publications:
Moisan, et al, U.S. Pat. No. 4,049,940 dated Sept. 20, 1977; Cobine, J. D., Wilbur, D. A., JOURNAL OF APPLIED PHYSICS, 22, 835 (1951); Zhukov, M. ed., THEORY OF THE ELECTRIC ARC, Norosibirsk 1977; Kapica, P., JETP, 57(6), 1801 (1969); Shogerbee, R. K. ANAL. CHEM. 48, 611A (1976); Laison, G. F., et al, ANAL. CHEM. 48, 1161 (1976); Boumans, P., de Boer, F. J., Holzel, H., Meier, A., SPECTROCHIM. ACTA, 30B, 449 (1975); Benakker, C. I. M., SPECTROCHIMICA ACTA, 31B, 483 (1976); 32B, 173 (1977); 33B, 53 (1978), and Moisan, M. C Beaudry and P. Leprince, IEEE TRANS PLASMA SCIENCE, PS-3, 55 (1975).
In all the devices referred to above, to avoid reflected power and achieve maximum energy transfer, additional tuning devices have been required. It has been found that when the devices of the prior art are used in the direct introduction or chromatography of gases and liquids, there is a tendency for any excess solvents present with the specimens being analyzed to extinguish the plasma.
Accordingly, it is the principal object of this invention to provide a microwave cavity of simple but efficient construction for the transfer of microwave energy to an ionizable gas to form a plasma.
It is a further object of this invention to provide an improved method of generating and maintaining a plasma adaptable for use in direct gas and liquid introduction and in gas and liquid chromatography.